Self-sorting in macroscopic supramolecular self-assembly via additive effects of capillary and magnetic forces

Supramolecular self-assembly of μm-to-mm sized components is essential to construct complex supramolecular systems. However, the selective assembly to form designated structures at this length scale is challenging because the short-ranged molecular recognition could hardly direct the assembly of macroscopic components. Here we demonstrate a self-sorting mechanism to automatically identify the surface chemistry of μm-to-mm components (A: polycations; B: polyanions) based on the A-B attraction and the A-A repulsion, which is realized by the additivity and the competence between long-ranged magnetic/capillary forces, respectively. Mechanistic studies of the correlation between the magnetic/capillary forces and the interactive distance have revealed the energy landscape of each assembly pattern to support the self-sorting results. By applying this mechanism, the assembly yield of ABA trimers has been increased from 30%~40% under conventional conditions to 100% with self-sorting. Moreover, we have demonstrated rapid and spontaneous self-assembly of advanced chain-like structures with alternate surface chemistry.

Electrochemical deposition of gold and platinum aggregates was conducted with an electrochemical workstation (CHI600E, CH Instruments, China). Morphology and elemental composition of gold and platinum were characterized with scanning electron microscopy (SEM) and energy dispersive spectrometer (EDS) (Supra55, Zeiss Microscopy, Germany). The magnetic strength of magnetic plates was measured with a gaussmeter (WT106, Weite Magnetic Technology Co., Ltd, China). Force measurements were conducted on a dynamic contact angle measuring device and tension meter (DCAT21) and water contact angle (WCA) were measured on an OCA20 instrument, both of which were from Data Physics Instruments GmbH (Filderstadt, Germany). Optical photographs and videos were taken with a Nikon camera (D7000, Japan).

Fabrication of platinum aggregates
The above Pt plates were prepared by a two-step electrochemical deposition (Supplementary Figure 1d) according to a previous report 1 . Firstly the glass slides sputtered with gold was pre-treated by immersing in a solution of thioglycolic acid (ethanol, 20 mM) for 2 h; secondly electrochemical deposition was conducted in a mixed solution of H2SO4 (aq, 0.5 mM) and HAuCl4 (aq, 1 mg/mL) for 1600 s (-200 mV) by using the above slides, a Ag/AgCl, and a Pt electrolde as the working, reference, and counter electrodes, respectively; afterwards, the plates were rinsed with deionized water and dried in nitrogen flows; thirdly the above plates were conducted with a second deposition following the same mode except changing to a mixed solution of H2SO4 (aq, 0.5 mM) and H2PtCl6 (aq, 1 mg/mL) for 2400 s (-200 mV), followed by rinsing and drying.

Modification of hydrophobic side surfaces of EPS
According to a reported method 2 , we have prepared a solution for the surface modification to obtain superhydrophobicity: (1) 1.0 g silicon dioxide (15 ± 5 nm) were dispersed in hexane and stirred for 3 h; (2) 1 mL of 1H, 1H, 2H, 2H-perfluorooctyltrichlorosilane was added and the resulted mixture was further stirred for 8 h, followed by settlement for half an hour; (3) after removing the supernatant, the solid was washed with ethanol for 3 times and dried at 60 °C; (4) the dried solid was dispersed in ethanol with a weight/volume ratio of 4 mg/mL. The purchased EPS cuboids were immersed in the dispersion for 5 min and dried in air at room temperature. The resulted water contact angle (WCA) of the modified EPS surface was 150.9° (Supplementary Figure 2c), proving the successful modification of superhydrophobicity.

Modification of hydrophilic side surfaces of EPS
The permanent magnets were embedded in PDMS by a sandwiched molding method (Supplementary Figure 3a); afterwards the entirety was modified with polyelectrolyte multilayers by a layer-by-layer (LbL) self-assembled technique (Supplementary Figure   3b). Firstly, the soft magnetic strips were cut into a hexagonal shape with a dimension shown in Supplementary Figure 1c and then aligned on a normal glass substrate, the four corners of which were adhered with four coverslips (thickness: ~0.6 mm) as spacers.
Secondly, a precursor mixture of PDMS containing pre-polymer, crosslinker and dye with a mass ratio of 10:1:0.1 were poured onto the aligned magnets. To distinguish the surface chemistry of PDMS for the subsequent LbL processes, we have dyed A building blocks as red and B building blocks as green in the PDMS precursor liquid. Thirdly, we covered these PDMS and magnets with another glass to form a sandwich structure, followed by heating at 65 °C for 4 h to crosslink PDMS. Finally, after de-molding we cut the PDMS into a dimension of 8 mm × 5 mm × 0.6 mm with the magnet embedded in the center. The 7 / 13 magnetic strength on the surface was measured to be about 5 mT with a gaussmeter.
Normal PDMS plates without magnets were prepared following the same procedure except for the step of adding magnets. In all experiments with magnetic forces, building blocks have north poles of magnets facing outward were marked as 'A' while building blocks have south poles facing outward were marked as 'B'. In control experiments without magnetic forces (Figure 4b), normal PDMS plates without embedding any magnets were modified similarly following the above LbL processes to obtain hydrophilic surfaces. In control experiments of 'all-hydrophilic' side surfaces (Figures 5d-e), the commercially available EPS were modified to be hydrophilic with the above LbL processes.

Supplementary Note 5: Measurements and Calculations of Capillary Forces
The capillary attraction was quantified by the following steps with hydrophilic side surfaces as an example. (1) Measurement of the critical distance to trigger capillary attraction of building blocks. We fixed A building block and released B building block at different 9 / 13 positions of varied distance (xd) from A (Supplementary Figure 5a). If A is located within the interactive distance of capillary forces, we observed fast movements of A towards B; otherwise, A kept static where we placed. As we changed the distance (xd=20, 21, 22, 23, 24, and 25 mm), we observed the movements of A at xd=22 mm, but A remained static at xd=23 mm, indicating a roughly critical distance of the capillary interaction to be 22 mm in our assembly system (Supplementary Figure 5b). (2) Calculation of the instantaneous force based on the motion trajectories of A in the above capillary attraction. We took a video to record the above capillary attraction between A and B and showed some snapshots in Supplementary Figure 5a. We measured the distance that A had moved after a certain time interval, e.g., st at t=t2; based on Newton's law of motion, we calculated the instantaneous moving velocity (vt) and the acceleration (at) at this moment; by neglecting the fluidic friction due to the low Reynold number, we roughly obtained the attractive force for the assembly, namely, the lateral capillary force (FMSA) shown in Figure   3a. The results of capillary forces obtained by the measurements of the critical interactive distance and the simulation via contour the functions have been co-plotted in Supplementary Figure 5c. The results matched well with each other when the two building blocks were away from a distance larger than 8 mm, but some deviation appears when they approached closer; the dramatically increasing trends of the forces values with the decreasing distance remain the same in both cases. The deviation may be caused by slight rotations (shown with arrows in Supplementary Figure 5a) contributed by wettability conflicts, which was not considered in the simulation due to model simplification and calculation. Theoretically, the capillary forces from both hydrophilic and hydrophobic side surfaces increase dramatically when the two building blocks approach into proximity, which increases the driving force together and favors for long-ranged alignment to realize precise matching between the interactive surfaces. Besides, complex fluidic dynamics may also influence the motions. magnetic attraction, which occurred when the two magnets contact each other, namely their interactive distance (xd) was zero. After calculations of converting the mass changes into the force changes and calibrating the interactive distance, we obtained the measured magnetic forces versus the interactive distance between MSA building blocks (Figure 3).